What unit is commonly used to measure altitude in aviation?

In aviation, altitude is most commonly measured in feet above sea level (ft ASL). This standard is widely adopted for several reasons, including ease of use in communication among pilots, air traffic controllers, and other ground personnel. The use of feet makes it easier to clearly differentiate altitude levels, especially when dealing with air traffic in crowded airspaces.

The aviation industry relies on feet ASL because it aligns with the historical practices of many countries, particularly in the United States and the UK. Despite metric measurements being used in many other fields, the tradition and clarity associated with using feet in aviation help ensure safety and effective communication.

In contrast, while meters above sea level can be used in some regions or contexts, they are not as prevalent in standard aviation operations and communications. Kilometers above ground level are less common as an altitude measurement in aviation, as pilots typically use feet for operational purposes. Inches of mercury, while important for pressure measurement and altimeter settings, do not directly define altitude but rather the atmospheric pressure required to determine it.